Subunit composition of nicotinic acetylcholine receptors from chick ciliary ganglia

Nicotinic ACh receptors were immunoaffinity-purified from chick ciliary ganglia, radioiodinated, and examined by SDS-PAGE. Components with Mr's of 49, 52, and 60 kDa were obtained. Limited proteolysis produced different peptide maps from the components, confirming the 3 as distinct species. All are glycoproteins since treatment with glycopeptidase F altered their migration during electrophoresis. The 60 kDa component appears to be encoded by the AChR alpha 3 gene since it was selectively immunoprecipitated by an antiserum to a fusion protein containing a putative cytoplasmic region of the predicted alpha 3 gene product. The 49 kDa component selectively cross-reacted on immunoblots with 4 monoclonal antibodies that recognize a component of similar size in AChR preparations from chicken brain. The 52 kDa component is a novel species not previously identified in preparations of brain AChRs. If all 3 components represent integral AChR subunits, they may compose 2 receptor subtypes in the ganglion, e.g., one containing 49 and 60 kDa subunits and another containing 52 and 60 kDa subunits. This is supported by the finding that a receptor preparation can be obtained containing only the 49 and 60 kDa components and is consistent with reports of brain AChRs having only 2 types of subunits. Alternatively, ganglionic AChRs may contain 3 or more types of subunits, with at least one being selectively lost under certain conditions.